The invention relates to a method of compensating for the cooling of an automatic parking brake and a system employing this method.
In a vehicle equipped with an automatic parking brake system, it is common practice for the brake to be brought into operation while the brake components are hot or even at a relatively high temperature. These components are therefore in an expanded state when the automatic parking brake system is applied.
In a vehicle where the parking brake acts on a disk brake system, it is the disk and brake pads which are hot. Following application of the automatic parking brake, when the vehicle is at rest, the disk and brake pads cool down and shrink. The braking force decreases in step with the contraction of the pads and disk.
This can result in a loss of braking pressure and can even release the vehicle. In some systems the braking force applied by the automatic parking brake takes this cooling into account and applies, while the brake device is hot, a braking force greater than that necessary to immobilize the vehicle. Some systems also provide for the brakes to be applied again after the brake disk and pads have cooled down.
In vehicles in which the automatic parking brake acts on a drum brake, heating has the effect of expanding the drum. As the drum cools, its volume decreases and the inside diameter of the braking surface decreases. After application of an automatic parking brake, and cooling of the drum brake, this decrease in diameter has the effect of increasing the braking pressure. The situation is therefore the reverse of that which occurs in systems where the automatic parking brake acts on a disk brake, because the braking force increases as the drum cools.
Automatic parking brake circuits usually have at least one function for indirectly measuring the braking force, for example by measuring the current supplied to the electric motor which actuates the parking brake system. In the absence of a direct force sensor capable of measuring the applied braking force, this force must be estimated by means of an indirect force measurement function (measuring the current supplied to the electric motor).
However, this measurement depends on several parameters (such as temperature, friction and the influence of speed). The braking force can therefore only be estimated in an approximate way.
Therefore, since it is vital that the braking force applied to the vehicle should not fall below a minimum under any conditions of braking and parking, it is acceptable to have a braking force greater than that theoretically necessary.